Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, a generator, a gearbox, a nacelle, and one or more rotor blades. The rotor blades capture kinetic energy of wind using known airfoil principles. For example, rotor blades typically have the cross-sectional profile of an airfoil such that, during operation, air flows over the blade producing a pressure difference between the sides. Consequently, a lift force, which is directed from a pressure side towards a suction side, acts on the blade. The lift force generates torque on the main rotor shaft, which is geared to the generator for producing electricity.
More specifically, the gearbox steps up the inherently low rotational speed of the rotor for the generator to efficiently convert the rotational mechanical energy to electrical energy, which is fed into a utility grid via at least one electrical connection. Gearless direct drive wind turbines also exist.
Some wind turbine configurations include doubly-fed asynchronous generators (DFAGs). Such configurations may also include power converters that are used to convert a frequency of generated electric power to a frequency substantially similar to a utility grid frequency. Moreover, such converters, in conjunction with the DFAG, also transmit electric power between the utility grid and the generator as well as transmit generator excitation power to a wound generator rotor from one of the connections to the electric utility grid. Alternatively, some wind turbine configurations include, without limitation, alternative types of induction generators, permanent magnet (PM) synchronous generators, electrically-excited synchronous generators, and switched reluctance generators. These alternative configurations may also include power converters that are used to convert the frequencies as described above and transmit electrical power between the utility grid and the generator.
At least some known electric utility grids include one or more series-compensated transmission lines. Sub-synchronous control interactions (SSCI) is a phenomenon that occurs when power-electronic converter controls interact with such series-compensated transmission lines. These interactions can sometimes lead to control instabilities if control systems are not tuned properly or if the control margin of the power converter in properly-tuned control systems is not maintained.
Accordingly, the present disclosure is directed to systems and methods for optimizing the utilization of DC voltage for electrical power systems, such as wind turbine power systems, so as to address the aforementioned issues.